Such sensors are well known in the art. As an example of such a sensor, a capacitive humidity sensor is considered in the next paragraphs.
Humidity sensing is of great importance in many industrial applications, including monitoring of pharmaceutical products, biomedical products, food items, beverage storage and logistics (by use of e.g., RFID-based sensor logging equipment), the automotive industry, semiconductors industry, etc. The acronym “RFID” stands for radio-frequency identification. For the food monitoring application for example, measuring the relative humidity inside the food package is necessary to monitor the quality of the product, as excessive moisture content will lead to premature spoilage. In such application, the miniaturization of the humidity sensors is also a key element. For these reasons, integrating the humidity micro-sensors using integrated circuitry (IC) technology, e.g., CMOS IC technology, decreases the cost of the processing, and gives an advantageous integrated solution.
The majority of the humidity sensors, commercially available nowadays, have a capacitive sensor configuration. The capacitor's electrodes are electrically isolated from one another by means of a suitable dielectric material. The material's dielectric constant has a value, which depends on the magnitude of the relative humidity. As a result, the capacitance depends on the relative humidity. Therefore, if the capacitance determines the functional behavior of an electronic circuit (e.g., RC-time, oscillating frequency, etc.), the monitoring of this behavior enables to determine the relative humidity.
The capacitor in a capacitive sensor can have one of several spatial configurations, taking into account the fact that the dielectric material should be exposed to the ambient air in operational use of the sensor.
A first configuration is referred to as the parallel-plate capacitor (or parallel plane capacitor) configuration. Herein, a dielectric layer, made from a thin polymer film or a metal oxide, is sandwiched between two metal electrodes. The upper electrode can be patterned or made porous, typically using gold (Au) so that water vapor in the air can reach the dielectric layer. The patterned electrode is configured to protect the dielectric layer from contamination and exposure to condensation. For examples of such a sensor, see, e.g., EP 0 395 349; “Integrated multifunctional humidity sensor”, S. A. Krutovertsev et al., Sensors and Actuators, A 62 (1997), pp. 582-585; “Modeling and Optimization of a Microscale Capacitive Humidity Sensor for HVAC Applications”, A. K. Sen et al., IEEE Sensors Journal. Vol. 8 (4), April 2008, pp. 333-340; EP 0 395 949; and “Improvement of polyimide capacitive humidity sensor by reactive ion etching and novel electrode design”, Y. L. Yang et al., IEEE Proceedings Sensors 2002, Vol. 1, pp. 511-514
Improvements to such sensor relate to, e.g., the design of the upper electrode, the adding of a heating element to control sensor sensitivity, the patterning also of the bottom electrode. As to the latter feature, openings are made in the dielectric material using the top metal grid of the upper electrode as a hard-mask. Thus, the area of the dielectric material exposed to the ambient air is increased. The bottom electrode is patterned as well, but is completely covered by the dielectric material. For more details see, e.g., “A high-sensitivity polyimide capacitive relative humidity sensor for monitoring anodically bonded hermetic micropackages”, M. Docmeki et al., Journal of Microelectromechanical Systems, (2001) Vol. 10 pp. 197-203.
A second configuration is referred to as an in-plane capacitor. In such a capacitor, the two electrodes are formed in the same layer on top of a substrate, and the tops of the electrodes are coated by the dielectric material. The electrodes may be shaped as a pair of interlocking combs. This electrode pattern is also referred to as interdigitated electrodes (IDE). Examples of capacitive humidity sensors using the in-plane configuration are disclosed in, e.g., U.S. Pat. No. 7,222,531; U.S. Pat. No. 6,742,387; U.S. Pat. No. 6,690,569; U.S. Pat. No. 6,222,376; U.S. Pat. No. 4,429,343; US 2002/0109959; and JP 2004-037405. The publication “High-Sensitivity Capacitive Humidity Sensor Using 3-Layer Patterned Polyimide Sensing Film”, J. Laconte et al., Proceedings of IEEE, Sensors, October, 2003, Vol. 1, pp 373-377, discloses an improved version of the IDE configuration. The capacitive humidity sensor considered in this publication is based on an IDE configuration and a polyimide sensitive layer. Aluminum interdigitated electrodes are formed with fingers of 1 μm width, and the distance between two adjacent fingers is 1 μm. The electrodes are deposited on a first insulating polyimide layer and are covered by two more polyimide layers. The upper polyimide layer features a regular array of holes to increase the active surface area.